Inheritance to Class (NSObject), it'll be the reference type.

InheritanceArray.swif

import Foundation

let apple: NSMutableArray = ["🍎", "🍎"]
var orange = apple
orange.replaceObjectsInRange(NSMakeRange(0, apple.count), withObjectsFromArray: ["🍊", "🍊"])

print("apple=\(apple)", "orange=\(orange)") // a=[🍊, 🍊] b=[🍊, 🍊]

InheritanceString.swift

import Foundation

let apple: NSMutableString = "🍎"
let orange: NSMutableString = apple

orange.setString("🍊")

print("apple=\(apple)", "orange=\(orange)") // a=🍊 b=🍊